For years food scientists have had a continuing interest in producing palatable, nutritious proteinaceous foodstuffs from plant and low-cost animal sources as a supplement to or as a replacement for animal protein foods. The proteinaceous product must be economical, nutritionally well balanced and aesthetically acceptable to the human consumer, i.e. the product must be bland in flavor and odor and must be free of objectionable colors. At the present time, such proteinaceous food products are principally prepared from leguminous or cereal proteins. Unfortunately, these protein sources are relatively deficient in some essential amino acids such as methionine, lysine and the like. Such amino acids are referred to by nutritionists as "limiting". As a result, the protein's nutritive value is reduced to the content of the first limiting amino acid, thus requiring excessive consumption of the protein to provide the proper nutritive value.
It is well known that the nutritive quality of a protein material may be improved by the addition of a calculated amount of the deficient or "limiting" amino acids. It has been shown, for example, that the addition of lysine to corn protein considerably improves the nutritional quality of that protein. Similarly, supplementation of soybean protein with methionine has resulted in the improvement of its nutritional quality. Unfortunately, however, not all attempts to fortify amino acid-deficient foodstuffs with the free amino acid have been successful in terms of customer acceptance because of severely intractable flavor problems that arise in part from the intrinsic nature of the particular amino acid added. In addition to the inherent flavor properties of the free amino acids, other unpalatable flavors arise from degradative processes such as air oxidation and particularly as a result of the Strecker degradation of the amino acid (a reaction yielding noxious compounds such as mercaptans and sulfides). Because of these adverse flavor problems, alternate means for alleviating the amino acid deficiency, particularly methionine deficiency, have long been sought.
Various methods for improving the flavor problems coincident with the addition of methionine and other amino acids have received considerable attention. Those methods suggested in the prior art include the encapsulation of the free amino acid in an inert media, such as beef tallow; the simultaneous incorporation of certain anti-Browning agents to defeat the Strecker degradation; the application of the plastein reaction; the utilization of various protein blends such as corn protein with soya protein; and the utilization of derivatives of methionine or other amino acids that are tasteless, non-toxic, stable and yet are nutritionally available on ingestion.
Of the above-mentioned methods, some modicum of success has been achieved with the use of amino acid derivatives. Examples of such amino acid derivatives are DL-methionyl-DL-methionine (disclosed in German Patent Application No. 2,251,877, published Apr. 25, 1974) and N-acyl derivatives of sulfur-containing L-amino acids (disclosed in U.S. Pat. No. 3,878,305). Although each of these derivatives shows some promise in the fortification of proteinaceous foodstuffs, each has drawbacks which limit their practical utility. Specifically, DL-methionyl-DL-methionine is bland in taste, but it is almost totally insoluble in water and dilute acid. Thus, this derivative is not useful in the fortification of many foodstuffs which require a water-soluble or mildly acid soluble form for addition, such as in beverages. The N-acyl derivatives of methionine, such as N-acetyl-L-methionine, have an inherent characteristic sour, acid flavor which limits its use for palatably fortifying foodstuffs. It would therefore be highly desirable to provide palatable proteinaceous foodstuffs which are nutritiously fortified with bland, water soluble forms of "limiting" amino acids, particularly methionine, and to provide a method for producing such palatable and nutritiously fortified proteinaceous foodstuffs.